Heretofore, as materials for cutting tools for cutting general steel, there has been used P-type cemented carbide according to the JIS (Japanese Industrial Standard) classification, such as alloy comprising a WC-Co alloy and 10 wt % or more of a carbide/nitride of Ti, Ta or Nb added thereto. Recently, along with an increase in cutting speed in cutting conditions, there has been increased the proportion of a coated cemented carbide to be used for the cutting tool material. Typically, such a coated cemented carbide comprises a substrate material of an M-type cemented carbide (e.g., an alloy comprising a WC-Co cemented carbide containing 5-10 wt % of a carbide/nitride of Ti, Ta or Nb added thereto), the surface of which has been coated with an about 3-10 .mu.m-thick ceramic layer (coating) comprising TiC, TiCN, TiN, Al.sub.2 O.sub.3, etc., by using a vapor-phase deposition process such as chemical vapor deposition (CVD) and physical vapor deposition (PVD).
However, in some cases, when the cemented carbide is covered with the above-mentioned coating, there has been posed a problem such that the coating layer itself comprising a brittle material is liable to provide a defect, or an .eta. phase (a general term for a decarburization phase comprising of Co.sub.3 W.sub.3 C, etc.) is produced at the surface of the substrate material of cemented carbide, thereby to reduce the coating strength. In order to prevent the occurrence of such a defect and/or .eta. phase, various attempts have been made to improve the cemented carbide as a substrate material to be coated.
For example, Japanese Patent Publication (KOKOKU) No. 7349/1984 (i.e., Sho 59-7349) discloses a substrate material for coated cemented carbide which has been caused to contain free carbon so as to suppress the occurrence of the .eta. phase, which is liable to appear at the surface of the cemented carbide substrate at the time of the coating. Further, Japanese Laid-Open Patent Application (KOKAI) No. 97866/1991 (i.e., Hei 3-97866) proposes a tool of coated cemented carbide comprising a substrate material of a cemented carbide having a low carbon content which has been coated by a CVD process using a reactant gas capable of hardly forming the .eta. phase as a starting material. Furthermore, Hisashi Suzuki, "Cemented carbides and Sintered Hard Materials" page 221, (1986) published by Maruzen K.K. (Tokyo, JAPAN) discloses a technique for preventing a decrease in the strength at the time of coating such that a .beta. phase-reduced layer (a phase or layer in which a composite carbide/nitride phase such as (W, Ti) (C, N) has disappeared) is formed on the surface of a substrate material for coated cemented carbide so as to increase the Co content in a surface region of the cemented carbide.
The formation of the .eta. phase can be prevented by utilizing the above-mentioned techniques disclosed in Japanese Patent Publication No. 7349/1984 and Japanese Laid-Open Patent Application No. 97866/1991. However, the thickness of the .eta. phase which has been prevented from being formed is as small as about 5 .mu.m, and therefore the effect thereof is not enough to prevent the propagation of a fatigue crack of 100 .mu.m or more which actually poses a problem in practical use of a cutting tool (Atsushi Fukawa, "Powder and Powder Metallurgy" 41 (1), page 3 (1994)). As a result, the actual service life of the resultant cutting tool is still short.
According to the above-mentioned technique for forming the .beta. phase-reduced layer in a cemented carbide substrate material, a region in which the binder phase content has been increased, can be formed in a thickness of about 20 .mu.m at the surface of the cemented carbide, so that the prevention of the initial breakage thereof can be expected. However, since the thickness of such a region is small in comparison with the length of the fatigue crack, the effect of suppressing the propagation of the crack is little. In addition, since the binder phase content at the surface of the cemented carbide is increased, the service life of the tool is rather shortened under a high-speed cutting condition, on the basis of a decrease in the resistance thereof to plastic deformation.
In general, the strength of cemented carbides having the same Co content substantially has a certain correlation with the WC particle size in the cemented carbide. As the WC particle size becomes smaller, the bending strength of the cemented carbide is increased, but the fracture toughness thereof is decreased. On the other hand, it is considered that when the bending strength is increased, a fine crack is less liable to occur, and that when the fracture toughness is increased, the propagation of a fine crack is retarded to a larger extent. Accordingly, in order to increase the service life of a tool, it is necessary to improve both of the strength and the toughness. From such a viewpoint, considerable efforts have been made to make such improvement.
As a cemented carbide having both of strength and toughness which has been improved by such developing efforts, for example, Japanese Laid-Open Patent Application No. 170451/1987 (i.e., Sho 62-170451) and U.S. Pat. No. 4,966,627 propose cemented carbides in which the hard phase such as WC comprises fine particles and coarse particles. However, these cemented carbides are not ones which have been developed while taking sufficient consideration of usage thereof as a substrate material for coated cemented carbide for cutting general steel, so that the optimization was insufficient in view of a cemented carbide substrate material, the surface of which is to be coated. As a result, in a case where these cemented carbide substrate materials are used for constituting a coated cutting tool, the performance of the resultant tool is still insufficient.
Further, Japanese Laid-Open Patent Application No. 255795/1993 (Hei 5-255795) describes a coated cutting tool in which WC as a hard phase constituting the cemented carbide substrate material is classified into three kinds, i.e., coarse particles, medium particles and fine particles, and the contents of these three species of particles are defined. In this coated cutting tool, an effect of suppressing the propagation of a crack is intended by defining the particle size distribution in WC in the above-described manner. However, since the particle size distribution is simply wider than that in the conventional product but is continuous, it is not expected that the resistance thereof to plastic deformation as a coated cutting tool is improved by reducing the binder phase content. In addition, in the cutting tool disclosed in the above Japanese Laid-Open Patent Application No. 255795/1993, since the WC fine particle content is high, the effect of suppressing the propagation of a crack is insufficient.
More specifically, in recent years, the cutting condition under which a coated cemented carbide is to be used becomes severer to a considerable extent. For example, in the case of high-speed cutting at a cutting speed of 300 m/min. or higher, the temperature at the tip of the cutting edge becomes extremely high. Accordingly, an improvement in the resistance to plastic deformation under such a high temperature is particularly demanded. In addition, in order to suppress the propagation of wear of a cutting tool, the cutting tool is often subjected to wet cutting wherein a coolant is used in combination therewith. Such wet cutting is particularly liable to cause breakage of the cutting tool. Accordingly, there has been strongly desired a coated cemented carbide capable of providing a tool which not only has an improved resistance to plastic deformation at a high temperature, but also is less liable to be broken under wet cutting.
An object of the present invention is to provide a coated cemented carbide which has solved the above-mentioned problems encountered in the prior art.
Another object of the present invention is to provide a coated cemented carbide which has been improved in strength and toughness while providing an appropriate balance therebetween.
A further object of the present invention is to provide a coated cemented carbide which has been improved in resistance to plastic deformation at a high temperature and is suitably applicable to a tool which is hardly broken during wet cutting.
A yet further object of the present invention is to provide a coated cemented carbide which is suitably applicable to a cutting tool which can exhibit good cutting performance in the cutting of general steel and a hard-to-machine material at a high speed and is capable of providing a longer service life of a tool.